Display technology using various schemes such as plasma display panels (PDPs), liquid crystal displays (LCDs) and the like, replacing cathode-ray tubes, according to the related art, has been proposed and has come onto the market based on recent developments in optical technology. The demand for polymer materials for the displays as described above has increased. For example, in the case of a liquid crystal display, as slimness, lightness, and relatively large screen areas are required, wide viewing angles, high amounts of contrast, suppression of a change in an image color based on a viewing angle and equalization of a screen display are particularly important issues.
Therefore, several types of polymer films have been used for polarizing films, retardation films, plastic substrates, light guard plates, and the like.
In liquid crystal displays, various modes such as a double domain twisted nematic (TN) mode, a super twisted nematic (STN) mode, an axially symmetric aligned microcell (ASM) mode, an optically compensated blend (OCB) mode, a vertical alignment (VA) mode, a multidomain VA (MVA) mode, a surrounding electrode (SE) mode, a patterned VA (PVA) mode, an in-plane switching (IPS) mode, a fringe-field switching (FFS) mode, and the like may be used. The modes as described above show respective inherent liquid crystal array properties and optical anisotropy followed thereby, depending on a liquid crystal type, and these may be main factors causing a relatively narrow viewing angle in a liquid crystal display. That is, in order to improve a viewing angle, liquid crystal optical anisotropy needs to be appropriately compensated for, and to this end, and a retardation film corresponding to a respective mode is required. Among the modes described above, in the case of a liquid crystal display in an IPS mode, since a horizontal alignment liquid crystal is used, optical anisotropy at an inclination angle in a non-driving state is not as great as compared to that in other modes, and therefore, it has been known that a comparatively wide viewing angle may be secured by only using an isotropic protective film. However, in this case, since compensation for an absorption axis of a polarizer at the inclination angle is not performed, degradation in contrast and modulation in color depending on a viewing angle may occur, and thus, in order to secure a wide viewing angle, a retardation film suitable for an IPS mode liquid crystal display should be used.
In order to perform the compensation for an IPS mode, a retardation compensation layer satisfying the condition of nx>nz>ny is required. Here, nx, ny, nz represent respective refractive indexes in respective film directions, and x, y and z directions respectively refer to a direction in which a refractive index is highest on a film surface, a direction in which a refractive index is lowest on a film surface, and a thickness direction. However, it is known that the retardation compensation layer satisfying the condition of nx>nz>ny may be difficult to be implemented while only using a monoaxially stretched film or a biaxially stretched film. Therefore, in order to form a retardation compensation layer satisfying the refractive index condition, several schemes for three-dimensionally controlling a refractive index, for example, a method of inducing excessive width contraction at the time of performing stretching using a shrink film according to the related art, a method of applying a strong electrical field to a stretched film, and the like, have been proposed, but a limitation in consecutively producing wide width films has been present, due to several technical and equipment problems to date.
Accordingly, as a retardation film for an IPS mode, a structure configured of a multilayer film having two or more layers has been practically suggested, and for example, structure such as A-plate/(+)C-plate, (+)B-plate/(−)C-plate, and the like may be used. In terms of materials, respective layers may be configured of a stretched polymer film, a liquid crystal organic coating layer, a retardation representation polymer coating layer, organic and inorganic composite materials, and the like. As a method of configuring such a multilayer composite film, several methods using, for example, multilayer extrusion, adhesion, direct coating, thermal lamination and the like, may be used. However, the implementation of the multilayer extrusion method has relatively significant technical and equipment difficulties in terms of adjusting respective layer retardation, and in the lamination method using an adhesive, films of respective layers need to be separately manufactured and then subjected to several processes such as adhesion, drying and the like, such that manufacturing costs are increased, and the occurrence of defects such as staining, local surface irregularities or mixing of foreign objects, and the like due to the use of an adhesive may increase.
In the case of the direct coating or high-temperature thermal lamination, a retardation value thereof may be easily altered due to a high temperature or erosion by an organic solvent or the like, depending on materials, and thus, a range of applications thereof is not very large, and in the case of direct coating, a usable base film has limitations. In order to directly coat a stretched film with a retardation material, solvent resistance of a base film may be more important, and in the case of a cellulose or olefin-based retardation film, since solvent resistance thereof is comparatively excellent, the method of directly coating a base film may be used, but a retardation expression range is different such that it may be difficult to use the films for an IPS mode. In addition, in the case of an acrylic or styrene-based retardation film, since such films may have little resistance to general-purpose organic solvents, for example, an aromatic hydrocarbon solvent such as toluene, xylene, or the like, a ketone-based solvent such as acetone, methyl ethyl ketone or the like, an organochlorine solvent such as dichloromethane, chloroform or the like, and the like, when the coating is performed using a coating agent containing the solvents as described above, since adhesion defects, along with increases in brittleness and defective retardation and staining properties may occur, there are significant limitations in selecting a coating agent. Therefore, research into, and the development of, a retardation film having excellent adhesion at an interface between an acrylic film and a coating agent while having excellent organic solvent blocking properties are required.